It's not really for us to tell you. It will depend on what your band sounds like, how hot
the pickups in your guitars are, what mics are used etc. Set it how it sounds good to you
in the room, bearing in mind that it will be brighter right in front of the cone. Have a
listen in the control room and tweak the amp if you need to. Some will say use less gain
than you normally would. That's often a valid way to go, but ultimately if you're a really
high gain sort of band, then you need a high gain sort of sound.

This all
said...There's a late 70's 100w JMP at that studio, according to their website. If it's a
nice one, and you're going for that Les Paul/Marshall Slash sort of a sound, it will most
likely be nicer than the DSL. Those JMPs were basically JCM800 circuits before the JCM800
came out, and they can be extremely good. The transformers are better sounding than
anything available to Marshall today. But it might not be such a great one...A lot of
those things have been heavily messed around with over the years. Try it.

I'd
also advise you to try separating your sounds a little between the two of you. You can try
the AC30 for the other guitar perhaps. OR try the JMP on one and the DSL on the other.
Maybe one guitar slightly lower gain.

Ultimately you need to work with your
engineer to get a sound that YOU like. Make a choice on the day and stick with it.

You can not change the sounds you record very much. You can bring out a bit of
midrange or clean up the bottom end but ultimately what you hear when you track is more or
less what you will hear on your record. Don't imagine that it will change much. If you're
not hearing something you like when the red light goes on, stop and work on it. There's
nothing wrong with spending far more time working on getting a sound than on actually
recording it if you're good players.